The present invention relates in general to a paper feeding apparatus for an image reader, e.g. such as an image scanner, and more particularly to a paper feeding apparatus ensuring the prevention of overlap feeding of sheets of paper.
The image reader, e.g. such as the image scanner, is provided with a paper feeder for feeding a sheet of paper set up in a hopper onto a pass line communicated to a reading portion. The paper feeder serves to deliver sheets of paper one by one from the top of a stack of the sheets of paper and is usually equipped with a mechanism for preventing overlap feeding of two or more sheets of paper due to the friction between the adjacent sheets. For example, JP-A-4-286558 discloses a paper feeder which is provided with a preventing mechanism of overlap feeding.
FIG. 20 is a schematic view of a main part of the paper feeder under the prior art as an example, which has substantially the same construction disclosed in JP-A-4-286558.
In FIG. 20, disposed on the starting end of the read line is a hopper 51 which is loaded with sheets of paper P such as manuscripts or various documents to be read. A pickup roller 52 is also disposed so as to rotate in contact with the surface of the top of the sheets of paper P. The hopper 51 is forced by a spring 51a toward the pickup roller 52 to press the surface of the top of the sheets of paper P against the pickup roller 52, to thereby make it possible to deliver only the top sheet of the paper P by the friction with the peripheral surface of the pickup roller 52.
On the exit side of the pass line from the hopper 51 three-stage conveyance rollers 53, 54 and 55 are arranged, for example, in order to feed the sheet of paper P to the image reading portion. The rollers nip and draw the sheet of paper P fed from the hopper 51 so as to convey them to downstream. Then, on the pass line between the hopper 51 and the first conveyance roller 53 the paper feeder is provided with a parting roller 56 and a retard roller 57 which works as a preventing mechanism of overlap feeding of the sheets of paper P.
The prevention of overlap feeding by means of the parting roller 56 and the retard roller 57 is well known in the field of image readers or copying machines. The retard roller 57 is disposed, by a torque limiter 57b, around a spindle 57a which rotates by a drive motor, not shown, in the direction indicated by the arrow in the figure. That is, by virtue of the provision of such a torque limiter 57b, when one sheet of paper P is delivered from the pickup roller 52 side, the retard roller 57 accepts the rotational torque from the parting roller 56 and then rotates in the conveyance direction of the sheets of paper P. When two sheets of paper P are fed and nipped in an overlapping manner, the retard roller 57 keeps its rotation in the direction indicated by the arrow, to put back the underlying sheet of paper P in an overlapping manner toward the hopper 51.
In addition, it is necessary to allow for the paper sizes from A3 in maximum to B5 in minimum, or smaller size, because the sizes of the sheets of paper P loaded on the hopper 51 are variously different. In cases where the paper feeder allows for various sizes of the sheets of paper P, the weight of the large sized paper P is heavier than the weight of the small sized paper P if both number of sheets of paper P exerted on the hopper 51 are the same. On the other hand, because the spring coefficient of the spring 51a is fixed, the large sized paper P makes the pressing force against the pickup roller 52 of the hopper 51 weak due to its heavy weight, whereas a smaller sized paper P tends to cause a stronger pressing force.
Also in the case where the pressing force against the pickup roller 52 varies depending on the size of paper P, an appropriate frictional force should be provided between the sheets of paper and the pickup roller 52, otherwise it is impossible even to deliver the sheet of paper P. For this reason, the spring coefficient of the spring 51a should be set so as to secure an appropriate frictional force between the sheets of paper P and the pickup roller 52 necessary for the delivery of the sheet of paper P in the case where a maximum number of maximum sized sheets of paper P are placed on the hopper 51.
In above case, it is possible to deliver the sheet of the paper P by the friction between the sheet of paper P and the peripheral surface of the pickup roller 52, whereas in case that the size of the sheets of paper P is small under above setting of the spring 51a, the weight is light, resulting in too strong force of the spring 51a. Accordingly, the reacting force by the pickup roller 52 toward the paper P will also increase, so that the friction force increase on not only between the pickup roller 52 and the top sheet of paper P but also between the sheets of paper. As a result overlap feeding of the adjacent sheets of paper P is often caused.
On the other hand, even though overlap feeding of the sheets of paper P has occurred, the parting roller 56 and the retard roller 57 disposed in close vicinity to the hopper 51 are originally expected to prevent overlap feeding by the retard roller 57 putting back the underlying sheet of paper P. However, if the pressing force of the pickup roller 52 against the sheets of paper P is too much excess, it will become difficult to put back the sheet of paper in overlap feeding by the putting back force of the retard roller 57. For this reason, the underlying sheet of paper P in overlap feeding is putted toward the paper feeding direction by the pickup roller 52 while simultaneously it is subjected to putting back force by the retard roller 57. As a result, jamming of the sheet of paper P may occur between the pickup roller 52 and the retard roller 57, making it impossible to deliver the sheets of paper P.
Another condition necessary for the separation of the sheets of paper P when the sheets of paper P are in overlap feeding, is that the critical setting torque of the torque limiter 57b is bigger than the sliding friction force between adjacent sheets of paper P. For example, if the sliding friction force between the adjacent sheets of paper exceeds the force of the critical setting torque of the torque limiter 57b when in particular the sheets of paper P have a rough quality and therefore the sliding friction coefficient is too big, the retard roller 57 may rotate in the paper feeding direction, disadvantageously permitting the occurrence of overlap feeding without separating the overlapped sheets of paper P.
Further, if the position of the retard roller 57 is fixed relative to the parting roller 56, a substantially constant pressing force is imparted to the sheets of paper P passing therethrough. Then, the frictional force between the contact surfaces of the overlapped sheets of paper P at the time in overlap feeding is substantially proportional to the magnitude of the pressing force by the parting roller 56 and the retard roller 57. Accordingly, under the conditions that the pressing force is strong with rough quality of sheets of paper P for example, the sliding frictional force between the adjacent sheets of paper P is apt to exceed the critical setting torque of the torque limiter 57b, resulting in the occurrence of overlap feeding.